Re: [PATCH v8 4/5] iommufd: Extend IOMMU_GET_HW_INFO to report PASID capability

[Nicolin Chen <nicolinc@nvidia.com>]

On Thu, Mar 20, 2025 at 03:57:26PM -0300, Jason Gunthorpe wrote:
> On Thu, Mar 20, 2025 at 09:47:32AM -0700, Nicolin Chen wrote:
> 
> > In that regard, honestly, I don't quite get this out_capabilities.
> 
> Yeah, I think it is best thought of as place to put discoverability if
> people want discoverability.
> 
> I have had a wait and see feeling in this area since I don't know what
> qemu or libvirt would actually use.

Both ARM and Intel have max_pasid_log2 being reported somewhere
in their vendor data structures. So, unless user space really
wants that info immediately without involving the vendor IOMMU,
this max_pasid_log2 seems to be redundant.

Also, this patch polls two IOMMU caps out of pci_pasid_status()
that is a per device function. Is this okay? Can it end up with
two devices (one has PASID; the other doesn't) behind the same
IOMMU reporting two different sets of out_capabilities, which
were supposed to be the same since it the same IOMMU HW?

Thanks
Nicolin